Wireless broadband speed issues

Hi all, I’ve just upgraded to Virgin Media’s 20mb broadband, it runs fine on my one pc which is connected to the router at about 18-20mb but my other pc which is connected wireless only runs at about 8mb. I have Scientific Atlanta DPX100 connected to my wireless router which my first pc is connected too by ethernet. The router is:

http://www.tp-link.com/products/product_des.asp?id=1

On my wireless pc I am using this wireless usb adapter:

http://www.play.com/PC/PCs/4-/3476884/-/Product.html?searchstring=Edimax+EW-7318Ug+%2f+Wireless+G+54Mbps+%2f+USB2.0+Network+Adapter&searchsource=0

Any ideas on why it would be running so slow?

Simon